You can enter the 'Friends' section by selecting the two-people icon on the lower left corner of your main screen.
At the top you will find a search bar, where you can find your new opponents.
Next to this bar, you will see the option of inviting friends. When doing so, the game will allow you to send invitations through the different apps you have on your device.
Finally, at the bottom of the screen you will be able to link your account to Facebook. This will allow you to play against your Facebook friends, and also use your social network's profile picture and username.